The present invention relates to an electrostatic recording type image forming apparatus such as a copying machine, a facsimile or a printer, and more particularly to a heat generating apparatus and a fixing apparatus using electromagnetic induction.
For an image forming apparatus such as a printer, a copying machine or a facsimile, in recent years, market demands for energy saving and an increase in a speed have been made more greatly. In order to achieve the performance of these demands, it is important that the thermal efficiencies of a heat generating apparatus and a fixing apparatus which are used in the image forming apparatus are to be improved.
In the image forming apparatus, an unfixed toner image is formed on a recording medium such as a sheet material, a printing paper, a sensitive paper or an electrostatic recording paper by an image transfer method or a direct method through an image forming process such as electrophotographic recording, electrostatic recording or magnetic recording. As a fixing apparatus for fixing the unfixed toner image, a fixing apparatus using a contact heating method such as a heat roller method, a film heating method or an electromagnetic induction heating method has widely been employed.
For the fixing apparatus using an electromagnetic induction heating method, JP-A-8-22206 has proposed a technique for generating Joule heat by an eddy current generated on a heat generating member to be a magnetic metal member by the magnetic field of induction heating means formed by an exciting coil and for causing the heat generating member to carry out electromagnetic induction heat generation.
Description will be given to the structure of the inverter power circuit of a fixing apparatus using an electromagnetic induction heating method according to the conventional art. FIG. 26 is an electric block diagram showing induction heating means and the inverter power circuit in the fixing apparatus using the electromagnetic induction heating method according to the conventional art.
In FIG. 26, a commercial power source 310 is connected to a rectifier diode 330 of a rectifying circuit for full wave rectification in an inverter power circuit 320, and is connected to either side of an exciting coil 220 in induction heating means 180 by connecting means 350 through a smoothing capacitor 340 of a smoothing circuit. Moreover, the commercial power source 310 is connected to a resonance capacitor 360 for resonance in parallel with the exciting coil 220 in the inverter power circuit 320.
Furthermore, either portion on the reverse side of the exciting coil 220 is connected to a switching unit (hereinafter referred to as an IGBT) 370 in the inverter power circuit 320 through the connecting means 350. Moreover, the IGBT 370 is connected to switching unit driving means 380 and is ON/OFF controlled in response to the control signal of a temperature control circuit 390. A DC power source 400 having a DC of 20V is connected to the switching unit driving means 380 through abnormal temperature detecting means (hereinafter referred to as a thermostat) in the induction heating means 180.
The IGBT 370 is turned ON/OFF so that a high frequency current flows to the exciting coil 220. A magnetic field is generated by the high-frequency current and an eddy current is generated in a heat generating member (hereinafter referred to as a heat roller) 130 to be a magnetic metal member provided opposite to the exciting coil 220, thereby generating Joule heat.
With such a circuit structure, the temperature of the heat roller 130 is controlled to be approximately 180° C. in a normal condition and both ends of a thermostat 210 are set in a short-circuit state.
When the temperature control does not act but a thermal runaway state is brought for some reason, the temperature of the heat roller 130 is rapidly raised so that the temperature of the thermostat 210 is also raised suddenly. When the rise in the temperature is continuously carried out so that the temperature of the thermostat 210 reaches 200° C. or more, both ends of the thermostat 210 are brought into an open-circuit state so that the supply of a power from the DC power source 400 to the switching unit driving means 380 is stopped. Since an output of the switching unit driving means 380 is pulled down, an input (hereinafter referred to as a gate) of the IGBT 370 is turned OFF when the supply of the power is stopped. Consequently, a current does not flow to the exciting coil 220 so that the heating over the heat roller 130 is stopped.
Even if the heat roller 130 reaches an abnormal temperature and this is detected by the thermostat 210 to stop the supply of a power from the DC power source 400 to the switching unit driving means 380, however, the line of the commercial power source 310 is not cut off but heating is further carried out due to the abnormality of the IGBT 370, resulting in fuming and ignition. Thus, there is a problem in respect of safety.
Moreover, it is necessary to change the induction heating means 180 and the inverter power circuit 320 depending on a supply voltage. In the case in which the induction heating means 180 is taken for the inverter power circuit 320 in manufacture, there is a problem in that the IGBT 370 in the inverter power circuit 320 breaks down.